CM9112 Datasheet, PDF (11/18 Pages) California Micro Devices Corp – Dual Inputs Dual Outputs High Accurate Fast Charger

PRELIMINARY
CM9112
Application Information (cont’d)
PD= (V_IN – V_OUT) ×I_OUT
In most cases, V_IN is fixed at about 5.0V (either the AC
adapter or the USB power input). The CM9112 has two
outputs; one for the charger and one for the system
from LDO1. The total power dissipation is:
PD= (5V – 4.2V ) ×I_SYS+ (5V – V_BAT ) ×I_FASTCHG
Highest power dissipation occurs when the battery at
its lowest level (3.2V), when it just starts in the Fast-
charge (CC) mode. Assuming V_IN = 5.0V, V_BAT = 3.2V,
I_CC = 1A, the PD = (5V-3.2V) x 1A = 1.8W. Assuming
Rth_(JA) = 50°C/W, then ΔT = 1.8W x 50°C/W = 90°C. If
the ambient temperature (T_A) is 35°C, then the junction
temperature (T_J) could reach 125°C without over-tem­
perature current foldback.
With over-temperature (OT) current foldback, the
CM9112 will throttle down the charging current, allow­
ing the junction temperature will reach steady-state
equilibrium of 105°C, which translates into 1.4W of
power dissipation, or 0.78A of charge current. As the
battery voltage rises during charging, the allowable PD
dissipation is increased. When the battery voltage
reaches 3.6V, a full 1.0A of charging current is allowed.
Dual-Level OTP and OCP
In addition to chip temperature regulation at 105°C, the
CM9112 provides absolute over-temperature shutdown
protection. In the case of a malfunctioning charger con­
trol, high ambient temperature or an unexpectedly high
IC thermal resistance, Rth_(JA) (for example; due to
faulty soldering of the charger IC chip), the power dissi­
pation from LDO1 alone could over-heat the device.
The CM9112 provides an absolute OTP shutdown at
junction temperature of 150°C.
Similarly, each output, LDO1 and VOUT (ISET2), has
its own current limit. ISET1 provides the total adapter
current limit for adaptive charging current control. How­
ever, in case of an inoperative ISET2 setting (for exam­
ple; R_ISET2 becomes shorted to ground), ISET1 will
function as backup over-current protection. Combining
the dual-level OTP and the dual-level OCP, the
CM9112 in effect provides four layers of protection
against charger or VSYS over-load faults.
The Need for OVP
There are two primary reasons for adding an input
OVP feature to the CM9112 charger. One is to protect
the charger and the host system when an adapter with
the wrong output voltage is plugged-in. The other is to
protect the charger IC and the system against input
surge voltage resulting from the ringing due to the input
capacitor and an inductive adapter power cord.
Almost all computer peripherals and consumer elec­
tronics use AC adapters. It is common to use an LCD
monitor, a printer, a laptop computer, an ADSL or cable
modem, a cell phone, an MP3 player, with all their indi­
vidual AC adapters clustered around a power strip. The
output voltages of these adapters vary, yet most of
these use a similar cylindrical style connector at the
device interface. Thus, the chance that a user might
plug-in a wrong adapter should not be taken lightly.
The CM9112 provides over-voltage protection (OVP)
against the plug-in of a wrong adapter, up to an output
voltage of 30V. The CM9112 drives a 30V P-type
power MOSFET as a disconnect switch. The propri­
etary OVP design of the CM9112 protects itself and the
host system against the intermittent connection of a
wrong adapter.
Another source of over-voltage comes from voltage
ringing that occurs when an adapter is first plugged-in,
as shown in Figure 4. A long power cord from the
adapter output can have an inductance of several µH,
and the input capacitor of a cell phone is typically a
10µF to 100µF ceramic, with very low ESR. Unfortu­
nately, the low resistance in the power cord and the low
ESR of the input capacitor provide little dampening to
this LC circuit, resulting in strong ringing, with input
voltage overshoot, when the adapter is first plugged-in.
The ringing could apply a peak voltage twice that of the
nominal adapter output voltage at the input capacitor
point.
The CM9112 can withstand several forms of OVP con­
ditions; DC, DC with ringing, or intermittent contact of
any frequency.
© 2006 California Micro Devices Corp. All rights reserved.
07/06/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 lTel: [phone] lFax: [phone] lwww.cmd.com
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